1. Field of the Invention
The present invention relates to a communication system including a fixed communication device and a portable communication device, for example, a keyless system for automobiles that is made up of an in-vehicle communication device and a portable communication device carried by a user.
2. Description of the Related Art
There is a communication system that wirelessly controls and monitors a remote target through data communication between a fixed communication device set up in the vicinity of the target and a portable communication device carried by a user. Communication systems of this type are, for example, remote engine start systems for vehicles and keyless entry systems, and have become increasingly popular in recent years. A remote engine start system is configured to remotely control the start, stop, and other actions of a vehicle's engine by operating a button or the like on a portable device. A keyless entry system is configured to transmit challenge data in a low frequency (LF) band from an in-vehicle communication device to a portable communication device when a switch or a touch sensor that is provided on a door handle or in a similar place is operated, other than when a button on the portable communication device is operated, and send a response signal in an ultra-high frequency (UHF) band from the portable communication device to the in-vehicle communication device in response to the challenge data. The keyless entry system executes the unlocking/locking of the door or other types of operation to control in-vehicle equipment in the case where this series of steps of authentication communication is established.
In vehicle communication systems of the type described above, one-way communication in the UHF band has commonly been used for remote communication from the portable communication device to the fixed communication device, but is being replaced with two-way communication due to a demand for higher sophistication of the functions of communication systems. Two-way communication enables the communication systems to deal with the diversity of pieces of equipment that are targets of vehicle control, and to provide data about vehicle condition to the user from the vehicle in a timely fashion.
One of methods of accomplishing two-way communication is to turn UHF communication that has been one way into two-way UHF communication. Another method of accomplishing two-way communication is to add a communication circuit compliant with a wireless standard of Bluetooth (trademark) Low Energy, or to replace a UHF communication circuit with a communication circuit that follows the wireless standard of Bluetooth Low Energy, and communication systems that employ this method are increasing in number.
The portable communication device needs to keep executing reception check operation all the time in a communication system that turns one-way UHF communication into two-way UHF communication. The portable communication device in a communication system that uses a Bluetooth Low Energy communication circuit to accomplish two-way communication also needs to execute communication operation intermittently even when the portable communication device is not connected for communication, in order to establish communication connection to the fixed communication device. Whichever method is used, accomplishment of two-way communication increases power consumption. Portable communication devices in general use a small-sized battery and accordingly have a problem in that increased power consumption shortens battery life. It is therefore indispensable to reduce the overall power consumption of a portable communication device. Methods of reducing unnecessary power consumption of a portable communication device in a stand-by mode have been proposed as a solution.
A keyless entry system disclosed in WO 2015/068727 A1 has a communication device configuration in which two-way communication means using Bluetooth Low Energy or other technologies is added to an existing keyless entry system. This system regards a state in which a portable communication device is not connected for two-way communication, or a state in which the signal strength is at a certain level or lower, as an unused state of a portable communication device, and suspends LF reception processing and all communication functions of the portable communication device other than that of a two-way communication unit when the portable communication device is in the unused state, to thereby suppress the increase in power consumption of the portable communication device.
Similarly to the keyless entry system disclosed in WO 2015/068727 A1, a portable device and a vehicle communication system disclosed in Japanese Patent Application Laid-open No. 2016-56598 have a communication device configuration in which two-way communication means using Bluetooth Low Energy or other technologies is added to an existing keyless entry system. In this system, a condition for putting communication units other than a two-way communication unit into operation is that the portable device is connected for two-way communication, or that operation performed on the portable device is detected, and a condition for suspending the communication units is that communication is cut off, or that a certain length of time has elapsed since the cutting off of communication. The same effect of suppressing the increase in power consumption as that in WO 2015/068727 A1 is expected.
The communication systems of WO 2015/068727 A1 and Japanese Patent Application Laid-open No. 2016-56598 both aim to keep power consumption low by constantly monitoring the connection state of the added two-way communication circuit and a communication state that is measured by signal strength or other indicators, and controlling other types of processing than two-way communication in a manner suited to the communication state. The premise of this approach is that the two-way communication circuit is much smaller in power consumption than other communication circuit units. The Bluetooth Low Energy communication method is attracting attention as a two-way communication method that requires very small power consumption in recent years, and is positioned as the main method of two-way communication in WO 2015/068727 A1 and Japanese Patent Application Laid-open No. 2016-56598 as well.
In Bluetooth Low Energy communication, however, a communication device that is to make connection for communication needs to transmit specific data periodically to the surroundings even while the communication device is not connected. It is therefore common in communication systems that employ the Bluetooth Low Energy communication method to execute communication processing in which a signal is transmitted periodically from the portable communication device to the fixed communication device. Power consumption can be reduced by setting a long interval for this communication. However, setting a long communication interval increases a delay in establishing communication since a request for communication is made, and results in poor response. This affects marketability, which means that there is a certain limit to how long an interval can be set for the communication. In the case where the communication interval is set to 1 second or so in consideration for responsiveness, power consumed by a two-way communication circuit of Bluetooth Low Energy is several times larger than power consumed by an LF band reception circuit and, even with an LF reception circuit unit suspended, the battery life of a portable communication device to which a two-way communication circuit of Bluetooth Low Energy is added is a fraction of the battery life in a portable communication device that does not use the two-way communication circuit. The resultant drop in marketability is a big issue.
Other than the patent documents cited above, there is a communication system in which UHF communication that has been one-way communication from a portable communication device to a fixed communication device is turned into two-way UHF communication for higher sophistication in function. This system, however, requires the portable communication device to remain open to reception in order to implement the two-way communication function, which cuts battery life short and requires frequent battery replacement. The system consequently has a problem of low convenience.